In general, a boron carbide sintered body is expected to have a wide range of applications as a material having a light weight and high hardness and being excellent in abrasion resistance or corrosion resistance. At present, it is used, for example, for a sandblast nozzle, a wire drawing die or an extrusion die. However, on the other hand, such a boron carbide sintered body has a drawback that it has low strength. For example, K. A. Schwetz, J. Solid State Chemistry, 133, 177-81 (1997) discloses preparation of boron carbide sintered bodies by HIP treatment under various sintering conditions, but a boron carbide sintered body having a flexural strength of at least 600 MPa has not yet been obtained.
Further, V. Skorokhod, J. Material Science Letter, 19, 237-239 (2000) discloses that a mixture comprising a boron carbide (B4C) powder, a titanium dioxide (TiO2) powder and a carbon (C) powder, is sintered under a pressurized condition employing a hot press method while reacting a part of boron carbide with titanium dioxide and carbon (see the following reaction formula), to obtain a boron carbide-titanium diboride sintered body, whereby a four-point flexural strength of 621 MPa is obtained.B4C+2TiO2+3C→2TiB2+4CO  Reaction formula
However, in order to make it practically possible to use a boron carbide based sintered body in a wide range of applications, it is desired to develop a boron carbide based sintered body having a still higher four-point flexural strength. However, as mentioned above, according to the conventional methods, a boron carbide based sintered body having a high four-point flexural strength exceeding 621 MPa has not yet been obtained.
Further, a boron carbide based sintered body is hardly sinterable and accordingly, it is usually prepared by a hot press method. This production method hinders a common application of a boron carbide based sintered body, since its production cost is high. Accordingly, it is being studied to prepare a boron carbide sintered body by heating (sintering) under a non-pressurized condition (a normal pressure method) instead of the hot press method. For example, in the above-mentioned prior art reference K. A. Schwetz, J. Solid State Chemistry, 133, 177-81 (1997), carbon is added as a sintering-assisting agent, and a boron carbide sintered body is prepared under a non-pressurized condition. However, such a method is not practically preferred, since it is necessary to carry out sintering at an extremely high temperature of at least 2150° C.
Further, a boron carbide sintered body has an extremely high hardness, whereby it can hardly be processed by a usual grinding/polishing method, and further, the electric conductivity of the boron carbide sintered body is low at a level of from 10 to 300 S/m, whereby there has been a problem that the discharge processing is difficult.
As mentioned above, a boron carbide sintered body is hardly sinterable and hardly processable, and at present, it is practically used only in an extremely limited application.
Under these circumstances, the present inventors have conducted an extensive research with an aim to develop a new boron carbide based sintered body which has a four-point flexural strength higher than the above-mentioned four-point flexural strength of 621 MPa and which makes it possible to realize a wide range of applications, and as a result, have found it possible to accomplish the desired object by selecting a specific material and by carrying out sintering treatment with a specific composition and under a specific temperature condition.
Further, the present inventors have found it possible to obtain a boron carbide based sintered body having excellent characteristics by preparing a sintered body having a specific microstructure wherein a highly electrically conductive chromium diboride phase forms a three dimensional network structure, by adding a predetermined amount of chromium diboride to a boron carbide powder having a specific physical property and carrying out liquid phase sintering under a non-pressurized condition to form a liquid phase of chromium diboride.
The present invention has been accomplished on the basis of the above discoveries.
Namely, it is an object of the present invention to provide a novel boron carbide based sintered body having a four-point flexural strength of at least 400 MPa and a fracture toughness of at least 2.8 MPa·m1/2.
Further, it is an object of the present invention to provide a boron carbide-titanium diboride sintered body having a four-point flexural strength of at least 700 MPa, preferably at least 800 MPa and a fracture toughness of at least 3.0 MPa·m1/2.
Further, it is an object of the present invention to provide a novel process for producing a boron carbide material which makes it possible to produce a boron carbide based sintered body which has a high density and having the fracture toughness improved, wherein the maximum particle diameter of boron carbide is at most 5 μm, the titanium diboride particles are uniformly dispersed in the boron carbide matrix, and the agglomerated/dispersed state of titanium diboride particles is uniform and good.
Further, it is an object of the present invention to provide a boron carbide based sintered body which has a relative density of at least 90%, an electrical conductivity of at least 5×102 S/m, a four-point flexural strength of at least 400 MPa and a fracture toughness of at least 3.0 MPa·m1/2, and a process for producing it by sintering under a non-pressurized condition.